The common fair in cities and dungeons. They are made of smelted metals each of which represents a different amount of money. Copper Pieces (CP) are worth the least. Many slums and similar areas use Copper Pieces as the common trading method.
Silver Pieces (SP) are the average coin. They are used as common currency in all cities around the world.
Gold Pieces (GP) are the worth the most. They are regularly used by governments and large expenditures (Such as Castles, Magic Items, and Soldiers). They are hard to find outside treasuries, vaults, ect. but sometimes bandits attack carvan transporting it (Which they normally store for a month or two so they don't arouse suspicion.

Dispite coins being common use in cities they are relatively useless in villages & towns, as they refuse to trade with something with no practical use. That is were Trade Goods come in.

To farmers, fishermen, and other serfs coins are worthless. They can't eat it, use it to help crops, protect there homes. To them it is useless shiny bits of metal. That is why they trade with objects for objects.

In the Nine Hells mortal souls are the only form of currency. Devils gain these souls by deals with the mortals in return for power, wealth, or anything the mortal wants. The mortal dies the soul travels to top layer of the Nine Hells to be retrieved by a Devil known as Kēphas. They then move the souls to there owner who -- after inspection of the souls -- sends them to a Kyton, who after a year of torture transforms the soul into a Lemure. This Lemure is then under the command of the rightful owner of the soul.